1. Field of the Invention
The present invention relates to a printing apparatus and image processing method, and particularly to a printing apparatus and image processing method that correct density unevenness.
2. Description of the Related Art
Ink jet printing apparatuses that are provided with a plurality of print heads or a plurality of nozzle arrays for ejecting ink of the same color are known. By providing a plurality of print heads or a plurality of nozzle arrays it is possible to achieve improved printing speeds. Printing apparatuses such as these, which are provided with a plurality of print heads or a plurality of nozzle arrays, however, often produce density or color unevenness in printed images. One cause of this is that an ejection characteristic difference between each print head or each nozzle array nozzle exists or is produced. Variation in the amount of heat generated by the heat generating heater, which is for ejecting ink, and variation in nozzle opening (ejection opening) diameter are raised as main causes of this kind of ejection characteristic difference between each print head or each nozzle array. There are also times when ejection characteristic differences are produced by fluctuations in the amount of heat generated by the heat generating heater, due to aging, or ink viscosity fluctuations, due to a change in the usage environment.
Calibration techniques are known as techniques to control density unevenness and the like caused by these kinds of ejection characteristic differences. These calibrations, for example, are carried out by changing the tables used in the y correction process that is performed as part of the image processing for correction of the ejection characteristics of the print head. Concretely, it is carried out by printing a patch on the print medium, detecting, from the resultant printed patch, the ejection characteristics of each print head or nozzle array at that time, and resetting the table used in the y correction process to a suitable object. As methods for detecting ejection characteristics based on a printed patch, there are methods of detection (inspection) of the printed patch by eyesight and methods of detection making use of input devices such as scanners and the like.
For example, in Laid Open Japanese Patent No. 2009-167947, a method of automatically carrying out correction (calibration) of density or color unevenness based on the result measured from establishing, in the carriage of the printing apparatus, a scanner or a light sensor for reading patches, and performing a density measurement of the printed patch via this scanner or the like. In this method, respective calibrations are carried out with respect to print heads of each ink color, and density correction values are obtained for each gradation of each ink color. Many of the previously known calibrations, as described in the above Laid Open Japanese Patent No. 2009-167947, are carried out in this manner with respect to respective print heads of each color of ink.
In contrast, as in the aforementioned printing apparatuses that are configured to be provided with a plurality of print heads, or a plurality of nozzle arrays of the same ink color, calibration is carried out with respect to a representative print head or nozzle array, and by applying the obtained density correction values to other print heads or nozzle arrays as well.
As mentioned above, however, regarding the case where ejection characteristic differences exist between each print head or each nozzle array, in setups in which calibration is carried out only with respect to a representative print head or nozzle array and the resultant obtained density correction values are applied to other print heads or the like, it is evident that a suitable density correction is not possible. In response to this, it is thought to obtain density correction values for each print head or each nozzle array. However, in the case where the utilization ratios of the plurality of print heads or nozzle arrays differ at each print area, the density correction amounts, which differ for each nozzle array, are expressed as densities according to the utilization ratios at each print area, and a new problem of generation of density unevenness among print areas is produced.
For example, in printing apparatuses that carry out printing by scanning a print head, which arranges nozzle arrays such that a plurality of nozzle arrays overlap each other in the direction that intersects the alignment direction of the nozzles, when the utilization ratios of the plurality of nozzle arrays used in printing differ at each raster, there are times when density unevenness, appearing as shade variation in a column direction in which the nozzles are aligned, occurs.
Also, for example, as for the so-called full line method printing apparatuses as well, in which a plurality of nozzle arrays are arranged in a scanning direction of the print head relative to the print medium, when the utilization ratios of the plurality of nozzle arrays used in printing differ at each column, there are times when shade density unevenness also occurs in a raster direction.